Mode control for lean burn engines

ABSTRACT

An intake system with multiple throttles, arranged in series, is disclosed for use in a lean burn, gasoline engine. The arrangement has the facility to provide air-fuel ratios which are: leaner than stoichiometric at the light load region of the operating map, stoichiometric at the high load region of the operating map, and richer than stoichiometric at the full load region of the operating map.

FIELD OF THE INVENTION

The present invention relates to mode control of a lean burn engine.Because lean burn operation can only be adopted in part of the operatingrange of an engine, even lean burn engines must on occasions be operatedin a stoichiometric or rich mode and the invention is concerned withmaking mode changes as imperceptible as possible to the driver.

BACKGROUND OF THE INVENTION

In lean burn engines, it is necessary under certain engine conditions tochange the fuel calibration from stoichiometric AFR (air to fuel ratio)to lean AFR or vice-versa. This may occur during driving when the enginespeed/load operating point is moved into or out of a lean calibrationwindow, and during lean cruise conditions when the engine AFR has to beperturbed briefly back to a rich AFR at regular intervals in order topurge a NOx trap in the exhaust system. The latter purge sequence couldbe very frequent, typically a 1 second rich excursion is required forevery 30 seconds of lean cruise running.

A well known control problem with lean burn engines is that the AFRchange can cause torque fluctuations which are unacceptable fordriveability. This arises from the fact that the intake air mass drawninto the engine is fixed and is set by the driver's pedal position at agiven vehicle speed. If the AFR calibration is to be suddenly changedagainst this fixed air mass, the fuel mass will change affecting theenergy produced and the engine torque. For example, a change in AFR fromstoichiometric to 22:1 represents a 35% drop in output torque at thesame air mass.

To compensate for this sudden change, the fundamental requirement isthat the intake air mass must in some way be changed at the same time asthe AFR is changed, so that the fuel mass in the engine remainssubstantially the same before, during and after the AFR change.

In one way of achieving this in the prior art, it is left to the driverto respond to the perceived change in torque by moving the demand pedalto a new position to change the intake air mass thereby regaining theengine torque. In effect, the driver response is in this case built intothe control loop, but this is only acceptable for small excursions inthe engine torque.

In another method disclosed in the prior art, an electrically controlledthrottle (ETC) is used to isolate the driver from direct interface withthe engine throttle. The driver sets the torque demand with apotentiometer which the ETC translates into a throttle positionprecisely matching the air mass required before and after the AFRchange. Thus during the AFR change, while the ETC rapidly moves thethrottle from one position to a new position to change the intake airmass, the driver who sets the torque demand does not feel any change inthe engine torque and therefore need not adjust his demand pedalposition. This AFR change, being totally transparent to the driver, isthen termed a seamless transition.

In WO96/21097, it is proposed to use an air dilution throttle inparallel with the main throttle and to gang the two throttles togetherto move at all times at the same throttle angle. An on/off valve isprovided in series with the air dilution throttle to enable or disablethe air dilution flow according to the lean or stoichiometric mode,respectively. This switching method has been demonstrated to produceseamless transitions similar to those using ETC, and has advantages overETC in that the ganged throttles are permanently connected to the demandpedal giving the driver direct control. Such a system has advantages oflower cost and higher reliability over the ETC system. It also lendsitself particularly well to the operating sequence of purging an NOxtrap by briefly flicking the on/off valve without moving the mainthrottle.

While the use of parallel throttles and an on/off valve in series withone of the two throttles is effective, it has disadvantages in that theair dilution throttle is a potential source of additional air leakagewhen both throttles are closed during engine idle operation, duringwhich only a very small amount of air leakage is permissible. This hasresulted in increased technical difficulties in the design of thethrottles because, even in the case of a single throttle, the total airleakage can attain a critical level. Furthermore other designconsiderations, in addition to the control of air leakage, for example,throttle effort, sludge and ice protection, fail-safe regulations etc.,that is applied to the main throttle must equally be applied to the airdilution throttle.

OBJECT OF THE INVENTION

The present invention seeks to mitigate the aforementioned problemsassociated with ganged throttles connected in parallel with one another.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an intake systemfor a lean burn engine comprising a first throttle connected to amanifold leading to the intake ports of the engine cylinders, a secondthrottle connected in series with and upstream of the first throttle andlinked for movement in synchronism with the first throttle, and a modecontrol means for changing between lean burn and stoichiometric modes byabruptly altering the pressure drop across the second throttle totransfer control of the effective through-flow cross-section of theintake system between the first throttle alone and the seriescombination of the two throttles.

In one embodiment of the invention, the mode control means is an on/offvalve connected in parallel with only the second throttle.

In an alternative embodiment of the invention, the mode control means isan override mechanism for temporarily disengaging the linkage betweenthe second throttle and the first throttle and fully opening the secondthrottle.

In contrast with the prior art method of connecting the air dilutionthrottle in parallel with the first throttle, the second throttle in thepresent invention is connected in series with the first throttle. Inthis case, the function of the first throttle is not affected in any wayby the addition of the second throttle so that all the stringent designspecifications for the intake system still remain satisfied within theexisting design of the first throttle. Moreover the design specificationfor the second throttle can now be relaxed to a large extent making thewhole system viable.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a block diagram of an intake system of a first embodiment ofthe invention,

FIG. 2 is a block diagram of an intake system of a second embodiment ofthe invention, and

FIG. 3 is a map of AFR against engine load to show the mode switchingbetween lean burn mode and stoichiometric mode in a lean burn engine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The intake system shown in FIG. 1 has a first throttle 10 which is themain throttle normally to be found at the air intake end of the intakemanifold. The first throttle 10 is connected to the demand pedaloperated by the driver and is associated with a throttle position sensor18. In the usual manner for a main throttle, a bypass passage 14 with anidle speed controller 16 is connected across the first throttle 10.

Upstream of the first throttle 10, a second throttle 20 and an on/offvalve 30 are mounted in an extension of the housing of the firstthrottle 10. The second throttle 20 is linked for movement insynchronism with the first throttle 10, the linkage being representedschematically at 22 by a dotted line. In the present embodiment, thelinkage 22 is arranged to move the two throttles through the samethrottle angle at all times, hence it may be formed of a gear system ora system of levers. The on/off valve 30 is associated with an actuator32 which may be an electric or a pneumatic motor for moving the on/offvalve 30 between fully closed and fully open positions. The size of theon/off valve 30 is such that when it is open, it effectively applies theambient atmospheric pressure to the first throttle 10 and the firstthrottle 10 alone determines the Through-flow cross-section of theintake system. When the on/off valve 30 is closed, on the other hand,the through-flow cross-section of the intake system is determined by theseries combination of the first and the second throttles 10 and 20.

The second throttle is sized smaller than the first throttle 10 so thatwhen it is brought into action by closing of the on/off valve 30, theair supply to the engine is abruptly reduced.

The intake system of FIG. 1 therefore operates in a manner analogous tothat disclosed in WO96/21097 in that if an on/off valve is operatedwhile the demand pedal is maintained in the same position, the air masssupplied to the engine undergoes an abrupt change. If the rate of fuelsupplied to the engine is correctly modified in synchronism with thechange in intake air mass, it is possible to switch between a lean burnmode and a stoichiometric mode without any perceptible change in enginetorque.

The operation of an engine fitted with the intake system of FIG. 1 canbe better understood with reference to FIG. 3 in which the calibrationof the relative air/fuel ratio (lambda) is plotted against engine loadfor a given engine speed. The complete calibration for the engine willcomprise several such maps at different engine speeds. The horizontalline at lambda 1 (partly solid and partly chain-dotted) that isdesignated Map 2 corresponds to stoichiometric mode operation. Theupwardly convex line designated Map 1 that peaks at lambda 1.5 (partlysolid and partly dotted) corresponds to lean burn mode and power modeoperations. If the calibrations of the maps 1 and 2 are correctlyperformed, then switching between the two maps (by following anyvertical line) at the same time as the on/off valve 30 is actuated willcause no change in engine torque.

The solid line portions of the two maps in FIG. 3 indicate the preferredcontrol strategy. In particular, the engine idles at stoichiometry,switches to lean burn during part load, reverts to stoichiometry atmoderately high load and eventually operates in a rich mode region ofMap 1 at full load. The reason for switching to Map 1 at full load isthat Map 2 relies on the second throttle 20 being effective which limitsthe breathing of the engine, whereas for Map 1 only the first throttle10 limits the breathing of the engine. It is for this reason that it isimportant that the sum of the areas A2 and A3 of the second throttle 20and the on/off valve 30 should exceed the area A1 of the first throttle10. The switching into the power mode can take place at a presetposition of the first throttle 10 as sensed by the throttle positionsensor 18.

The above strategy achieves smooth running during idle, improved fueleconomy during part load, and maximum performance at high load.Furthermore during lean burn operation, one can briefly flick into astoichiometric or rich mode to purge an NOx trap in the exhaust system.

In calibrating the lean burn Map 1 on an engine dynamometer to maintainconstant torque during mode changes, the computed fuel will not onlycompensate for the change in the intake air mass caused by switching theon/off valve 30, but will also take into account lesser effects such assimultaneous or consequential changes in manifold vacuum, pumping work,thermal efficiency, spark timing, exhaust gas recirculation etc.

In common with the proposal in WO96/21097, a simple mechanism isprovided to achieve seamless mode changes. If, at the same time asoperating the on/off valve 30, the fuel calibration is changed byswitching between Map 1 and Map 2, then regardless of the prevailingload and speed conditions of the engine, the mode change will not beperceived by the driver who will not need to modify the demand pedalposition in any way as a consequence.

The advantage of the system of the present invention over the proposalin WO96/21097 is that the tolerance required in the second throttle 20and the on/off valve 30 is not as great as that required in the firstthrottle 10. The reason for this is that when the on/off valve 30 isopen, the upstream pressure at the first throttle 10 is ambient pressureand it is of no importance if leakage occurs past the second throttle20. When the on/off valve 30 is closed on the other hand, as would bethe case during idling, air leakage past the second throttle 20 will notaffect the idle speed which still remains under the control of the idlespeed controller 16 across the first throttle 10.

Indeed it is desirable intentionally to reduce the tolerancerequirements on the second throttle 20 and the on/off valve 30 to avoidicing, sludge and other causes of jamming. This not only improvesreliability but reduces manufacturing cost.

The embodiment of FIG. 2 in terms of the air flow to the engine isidentical with that of FIG. 1 but instead of opening an on/off valve 30in parallel with the second throttle 20 when it is desired to disablethe second throttle 20, the second throttle is itself moved to a wideopen position achieving the same objective of applying the ambientpressure upstream of the first throttle 10.

The first throttle 10, the bypass passage 14, the idle speed controller16 and the throttle position sensor 18 in FIG. 2 are the same aspreviously described by reference to FIG. 1. The second throttle 20' isof a larger diameter than the second throttle 20 of the first embodimentand is connected to the first throttle 10 by a modified linkage 22'. Inthis embodiment, the first and second throttles 10 and 20' are not movedby the same throttle angle, the second throttle 20' being turned througha lesser angle to achieve the same through-flow cross-section as that ofthe smaller second throttle 20 in FIG. 1.

In FIG. 2, an override mechanism allows the second throttle 20' to bemoved to a wide open position whenever desired. The override mechanismcomprising a lost-motion coupling 24 with a stop that defines thepartially closed position set by the linkage 22' in one direction whileallowing the second throttle 20' to be opened fully in the oppositedirection. The actuating motor 32' will in this case either bias thesecond throttle 20' towards the partially closed position set by thelinkage 22' or to the wide open position depending on the stoichiometricor lean mode of operation respectively.

The maximum through-flow cross-section A3 of the second throttle 20'when it is fully open must exceed the through-flow cross-section A1 ofthe first throttle 10 in order not to limit the breathing of the engineat full load.

What is claimed is:
 1. An intake system for a lean burn enginecomprising a first throttle connected to a manifold leading to theengine, a second throttle (20) connected in series with and upstream ofthe first throttle (10) and linked for movement in synchronism with thefirst throttle (10), and a mode control means (30,32) for changingbetween lean burn and stoichiometric modes by abruptly altering thepressure drop across the second throttle (20) to transfer control of theeffective through-flow cross-section of the intake system between thefirst throttle (10) alone and the series combination of the twothrottles (10,20).
 2. An intake system as claimed in claim 1, whereinthe engine is provided with two air/fuel ratio calibration maps, thefirst map for use when the first throttle controls the effectivethrough-flow cross-section of the intake system, and the second map foruse when the series combination of the first and second throttlescontrols the effective through-flow cross-section of the intake system,the air/fuel ratio settings of the two maps at the same operating point(same speed and load) on the two maps being such that the output torqueof the engine remains the same during a mode change.
 3. An intake systemas claimed in claim 2, wherein the air/fuel ratio setting on the firstmap is calibrated at leaner than stoichiometry over the part load regionof the map and is ramped towards stoichiometry at the high load regionof the map and is richer than stoichiometry at substantially the fullload region of the map and wherein the air/fuel ratio setting on thesecond map is calibrated at stoichiometry over substantially the entireregion of the map.
 4. An intake system as claimed in claim 2, whereinthe calibration maps in addition to allowing for the change in air flowduring mode changes, take into account changes in other parametersaffecting the output torque of the engine.
 5. An intake system asclaimed in claim 4, wherein the mode control means includes an on/offvalve (30) connected in parallel with only the second throttle (20). 6.An intake system as claimed in claim 5, wherein the maximum through-flowcross-section of the second throttle (20) is smaller than the maximumthrough-flow cross-section of the first throttle (10).
 7. An intakesystem as claimed in claim 6, wherein the combined maximum through-flowcross-section of the second throttle (20) and the on/off valve (30) whenit is open is at least equal to the maximum through-flow cross-sectionof the first throttle (10).
 8. An intake system as claimed in claim 4,wherein the mode control means comprises an override mechanism (32') fortemporarily disengaging the linkage between the second throttle (20')and the first throttle (10) and fully opening the second throttle (20').9. An intake system as claimed in claim 8, wherein the maximumthrough-flow cross-section of the second throttle (20') is at leastequal to the maximum through-flow cross-section of the first throttle(10).
 10. An intake system as claimed in claim 1, wherein the first (10)and second (20,20') throttles are butterfly throttles.
 11. An intakesystem as claimed in claim 5, wherein the second throttle (20) is gangedwith the first throttle (10) for movement through the same throttleangle.
 12. An intake system as claimed in claim 8, wherein the secondthrottle (20') is linked with the first throttle (10) for movementthrough a smaller throttle angle than the first throttle (10).
 13. Anintake system as claimed in claim 1, wherein the mode control means isactuated electrically or pneumatically.